Power meters are used to measure consumption of electrical energy in households, companies or industry. Most power meters generate pulses on a so-called active energy kWh-LED. Each generated pulse relates to a predefined energy amount which is consumed by the load. The number of pulses generated per kWh is referred to as the pulse number “imp” which is typically between 1,000 and 100,000.
Before selling the power meters, manufacturers perform power meter calibration. FIG. 1 shows a possible configuration for a power meter calibration and testing of a power meter 1. A calibration equipment 2 supplies power with an alternating voltage UAC. It also controls a load current IL using an adjustable load 3 connected to the power meter 1. The power meter 1 measures the load current IL, the supply voltage UAC and uses these measurements to calculate an active energy consumption using formula:EA(t)=∫UAC(t)IL(t)dt  EQ.1
where EA(t) is the active energy consumption at a time t.
Besides the calculation of the active energy consumption, the power meter 1 also transforms consumed energy consumption into a pulse stream on a LED 5, also referred to as the kWh-LED 5. For the purpose of calculation metering quantities and pulse generation, the power meter 1 comprises a microcontroller 8 and an analogue front-end (AFE) 9. The microcontroller 8 of the power meter 1 may also be arranged for driving human machine interfaces, communication, and controlling other tasks.
The calibration equipment 2 reads pulses generated by the power meter 1 through an optical head 6 and performs its back-transformation into an amount of consumed active energy. The calibration equipment 2 then compares the active energy consumption registered by the power meter 1 with the reference energy and calculates an energy error in percents. Energy errors for particular load points are documented in a power meter calibration protocol. The calibration protocol may comprise active energy errors measured in a variety of load points in four quadrants of operation.
The generated pulse output provided by the kWh-LED 5, driven by the power meter 1 can be jittery due to the presence of the 100 Hz component caused by the multiplication of the instantaneous 50 Hz voltage and current waveforms. As a consequence the active energy calibration may take longer. The longer the calibration time, the higher the cost of the production and the lower the production throughput. In an effort to lower the production cost and increase manufacturing bandwidth, it is necessary to shorten power meter test and calibration time. Conventional software based pulse output techniques prolong calibration time, hence cheaper and better solutions are needed. An ideal power meter generates pulse output with a frequency in the very high-dynamic range. For example, a power meter with 50,000 pulses/kWh, measuring phase currents in the range from 20 mA (starting current) up to 150 A (maximum current) and operating at voltages in the range from 207 V (Un−10%) up to 253 V (Un+10%), will toggle the kWh-LED 5 with frequencies ranging from 0.0575 Hz up to 527.083 Hz. The accuracy of the pulse output shall be ideally better than the accuracy of the calibration equipment (0.01%).
These pulse output requirements can be accomplished using a dedicated high-resolution (>=27-bit) pulse output hardware (ASIC) which is an expensive solution. Apart from dedicated pulse output analogue ASICs, the pulse output can be handled fully digitally by the microcontroller 8 of the power meter 1, or any other type of processor. The microcontroller 8 may receive a supply voltage UAC and a load current IL from the AFE 9. The digitized quantities may further be processed by software in a software loop or in a dedicated software task. The software may updates an active energy counter whenever new supply voltage UAC and load current IL samples can be read from the AFE 9. After the active energy counter is updated, its value is compared with defined pulse output threshold values; if its value crosses a next threshold then the software toggles an active energy pulse output. Hence the pulse output update rate equals the AFE output sample rate fout. The inverse value of AFE output sample rate is referred to as the calculation step Tc=1/fout, also known as numerical integration step.
FIG. 2 shows a graph relating to a prior art method of generating a so-called “Rough Pulse Output” signal, see line 21. The method may be performed in a software loop or a dedicated software task executed on the microcontroller 8 of the power meter 1 or any other programmable logic device arranged in or in communication with the power meter 1.
In FIG. 2 the y-axis indicates the energy level EA where Lk is referring to an energy level equal to k*Th with k=0, 1, 2, . . . and the threshold Th being equal to 1/imp kWh. FIG. 2 shows a line 22 indicating the function of the active energy consumption EA(t) that is controlled by the calibration equipment 2 through adjustments of the load 3. The calibration equipment 2 generates a “Reference Pulse Output” signal 23 that represents instantaneous energy consumption. Contrary to the time-accurate “Reference Pulse Output” signal 23, the power meter 1 software generates the “Rough Pulse Output” signal 21 at calculation steps [n−1, n, . . . ].
In FIG. 2 a time gap ΔTref is referring to the time between a first pulse and a second pulse of the “Reference Pulse Output”, and a time gap ΔTrpo is referring to the time between a first pulse and a second pulse of the “Rough Pulse Output”. The cycle-by-cycle jitter of such software generated “Rough Pulse Output” can be expressed as:Rough_Pulse_Output_Jitter ≃ΔTref−ΔTrpo∈0,Tc  EQ.2
where Tc=1/fout is the calculation step and fout is the output sample rate of the AFE.
FIG. 3 shows a flow chart illustrating a method 30 of “Rough Pulse Output” generation according to the state of the art. The “Rough Pulse Output” is asserted with a resolution of the calculation step Tc.
The method 30 starts with reading phase voltage UAC and phase current IL samples from the AFE in a step 32. Then in a step 33, an energy contribution value ΔE is calculated. Next in a step 34, an energy value E[n] is calculated by adding a reminder value E[n−1] to the calculated ΔE value. Note that ‘n’ stands for the calculation step number of the present calculation step.
In a first test step 35, the energy value E[n] is compared to a positive threshold value (i.e. +1*Th). If the energy value is higher than the positive threshold value, the method continues with a step 36 that sets the temporary variable tmp to the positive threshold value, and next a step 39 in which a “Rough Pulse Output” signal is set to 1, and finally a step 41 where E[n−1] is calculated as energy value E[n] minus the temporary variable tmp.
If the first test step 35 is evaluated as false then execution of a second test step 37 begins. In the second test step 37, the energy value E[n] is compared to a negative threshold value (i.e. −1*Th). If the energy value E[n] is lower than the negative threshold value, the method continues with a step 38 that sets the temporary variable tmp to the negative threshold value, and next in step 39 the “Rough Pulse Output” signal is set to 1, and finally step 41 where E[n−1] is calculated as energy value E[n] minus the temporary variable tmp.
If none of the test steps 35 and 36 are evaluated as true then the reminder E[n−1] is set to the energy value E[n], see step 40. The method 30 is repeated in every calculation step Tc. As will be clear to the skilled reader, the pulse output will be set back to zero, but this clearing of the “Rough Pulse Output” signal is not relevant for understanding the principle hence not shown in the flow chart of FIG. 3.
The state of the art method described above can be executed using a software pulse output generator having a relatively simple implementation. It allows pulse output generation in a high dynamic range, but a relative long time is required for pulse averaging to eliminate the pulse output jitter.
The impact of pulse-per-pulse jitter on energy accuracy measurement can be explained by way of the following measurement configuration: power meter impulse number=50,000 pulses/kWh, input supply voltage UAC=253 V, load current IL=150 A and calculation step Tc=833.3 μs. The pulse output rate will equal to 527.083 Hz for given conditions. In order to guarantee accuracy of the active energy measurement in a range +/−0.01%, the calibration equipment 2 must continue averaging “Rough Pulse Output” pulses, which are generated with 833.3 μs jitter, for at least 4392.2 pulses, i.e. 833.3e-6/((1/527.083)*0.01/100). Hence a minimum test time for the conditions above is equal to 8.33 seconds.
The longer the testing of the power meter 1, the higher its manufacturing costs and the lower the factory production bandwidth.